Maternal insulin resistance initially remains unchanged from non-pregnant values or slightly lower before 14 weeks’ gestation; thereafter, insulin resistance progressively elevates to peak levels around 34 weeks’ gestation, as shown in Figure 1.
This decline in insulin sensitivity arises from increased maternal adiposity, placental growth hormone, and other factors. (Catalano et al., 1991; Catalano et al., 2014; Kampmann et al., 2019; Usman et al., 2023) The resulting changes in available substrates and finer tuned controllers determine the composition of the fuel (glucose versus fatty acids) driving mitochondrial energy production. (Randle et al., 1963; Hue at al., 2009; Jeon et al., 2016; Wasserman., 2022; Pappas et al., 2023; Gonzalez-Rellan et al., 2023)
Glucose and fatty acids differ in the metabolic pathways involved in energy production (Figure 4).
For example, glycolysis generates pyruvate, which is converted to acetyl-coenzyme A (CoA). Acetyl-CoA supports the tricarboxylic acid (TCA) cycle, which, in turn, provides the energy that drives mitochondrial ATP production. In contrast, fatty acids are transported to the mitochondria where sequential β-oxidation of 2C units of acyl-CoAs yields one acetyl-CoA and one ATP per cycle. (Lopaschuk et al., 2016; Batchuluum et al., 2018; Houten at al., 2016)
The increased insulin resistance displayed during the second and third trimesters results in a greater contribution of fatty acids to maternal mitochondrial ATP production. By reducing maternal utilization of glucose, this metabolic adaptation promotes placental transfer of glucose from mother to the fetus, which provides critical substrate support for fetal metabolism and growth.
The maternal pancreatic β-cells normally increase insulin secretion to counter the rise in insulin resistance after 14 weeks’ gestation. In GDM, this β-cell compensation is insufficient to counter the gestational fall in insulin sensitivity. (Usman et al., 2023) The ensuing inefficiency in maternal glucose utilization increases maternal 1) insulin requirements, 2) energy dependence on fatty acid substrates, and 3) circulating blood glucose levels.
T2D is associated with an impairment of L-carnitine and acyl-carnitine entry into mitochondria. The resulting elevation in circulating levels signal a dysfunction in mitochondrial oxidation of fatty acids. (Batchuluum et al., 2018; Reuter et al., 2012)
Both early-onset T2D and GDM are associated with higher circulating concentrations of medium-chain (C6-C12) acyl-carnitines. These lipid metabolites diminish both 1) insulin sensitivity and 2) the glucose-stimulated insulin release by pancreatic β-cells and thus contribute to the dysregulation of maternal glycemia after midgestation. (Batchuluum et al., 2018; Roy et al., 2018)
The current study revealed that in late gestation half of the eight consensus metabolites independently linked to GDM involved disruptions in fatty acid catabolism. These included a greater urinary excretion of one ketone (3-hydroxybutyrate), one small chain (<6C) acylcarnitine ester (3-hydroxy-butyrylcarnitine), one medium-chain (6-12C) acylcarnitine (3-methylhexanolylcarnitine), and one medium chain fatty acid metabolite (3-hydroxydodecanedioate). Not surprisingly, fatty acid derivatives were dominant constituents of the predictive model.
4.1 Amino acid metabolism. Lysine catabolism generates saccharopine and 2-aminoadipate. (Scislowski et al., 1994) In nonpregnant subjects elevated fasting levels of plasma 2-aminoadipic acid raised the long-term risk of T2D, (Wang et al., 2013) and in late pregnancy plasma lysine levels correlated positively with GDM. (Park et al., 2015) In the present study urine saccharopine excretion was simultaneously positively associated with GDM, as observed for first-trimester urine. (Koos and Gornbein, 2021) (Table 2) The elevated urinary excretion of saccharopine is consistent with a GDM-associated rise in hepatic and/or renal metabolism of lysine. (Gatrell et al., 2013)
4.2 Inflammation and oxidation. Hyperglycemia and elevated free fatty acids trigger the release of pro-inflammatory mediators and oxidative stressors, which, in turn, promote insulin resistance and impair insulin release by pancreatic β-cells. (Usman et al., 2013; Perry et al., 2015; Plows et al., 2018; Lee et al., 2018; Pinto et al., 2023)
Cohort studies indicate that GDM subjects express proinflammatory perturbations in the first trimester, such as increased plasma cytokines, gut microbial dysbiosis, (Pinto et al., 2023) and characteristic metabolite perturbations in urine. (Koos and Gornbein, 2021) These observations are consistent with the involvement of proinflammatory mediators in early and late gestation. (Kirwan et al., 2002; Piao et al., 2019; Ye et al., 2023)
GABA is expressed in the brain and other tissues, including pancreatic beta cells. (Vincent et al., 1983; Tillakaratne et al., 1995) GABA combines with histidine to form the dipeptide homocarnosine, an antioxidant, anti-inflammatory compound, and an inhibitory neurotransmitter. The inclusion of homocarnosine in the algorithm is consistent with the GDM-associated metabolic disruptions related to increased oxidative stress/inflammation. (Teufel et al., 2003; Peters et al., 2015; Huang et al., 2018)
Plasma acetylcholine derives largely from non-neuronal tissues, including the placenta. ( King et al., 1991; Wessler et al., 2001; McLatchie et al., 2021; Wessler et al., 2003) In the current study, urine acetylcholine excretion was positively and independently associated with GDM. Circulating acetylcholine is a marker of inflammation, (Cox et al., 2020) and the raised renal excretion in GDM subjects in late pregnancy is consistent with heightened placental inflammation in this disorder. (Pen et al., 2021)
4.3 Renal glucose excretion. Removing one hydroxyl group from glucose results in 5-anhydroglucitol, a diet-sourced compound that is insignificantly metabolized by human tissues. (Dworacka et al., 2006; Kim et al., 2013; Pramodkumar., 2019) Blood levels of 5-anhydroglucitol are independent of prandial state, body weight or age. Because of renal tubular reabsorption, little of this compound undergoes urinary elimination.
Glucose competes with 5-anhydroglucitol for the renal tubular transporter. In type 2 DM the augmented glucose delivery to the renal tubules reduces renal tubular uptake of 5-anhydroglucitol and thereby 1) increases urinary excretion and 2) reduces circulating levels. Low serum levels of 1-5-anhydroglucitol are markers for intermediate- or short-term hyperglycemia in type 2 DM and are a modest predictor (ROCAUC ̴0.69%) of GDM. (Ma et al.,2017)
4.4 Comparison Accuracy of prediction model. The computed algorithm derived
from urine metabolites independently linked to GDM in late pregnancy had a high accuracy in separating GDM versus CON (ROCAU 0.923; accuracy 89.1%). The metabolite model previously derived from early pregnancy urine similarly had a high discriminatory power (ROCAU 0.88, accuracy 83.9%).